compile: make Regex::new(r"(?-u:\B)") fail again

This regex failed to compile in `regex <1.8`, but the migration to
regex-automata tweaked the rules in a subtle way that permitted it
to compile despite the fact that the old/status-quo matching engines
can't handle it correctly. By that, I mean that they may permit the \B
to match between code units. That in turn results in panicking when
slicing a &str.

In `regex 1.9`, this regex will actually be able to be compiled, but
the matching engines will correctly and robustly never report matches
that split UTF-8 code units. For now, we just add code that causes
`regex 1.8` to have the same behavior as previous releases.

Fixes #1006

src/compile.rs

@@ -137,6 +137,15 @@ impl Compiler {
     }
 
     fn compile_one(mut self, expr: &Hir) -> result::Result<Program, Error> {
+        if self.compiled.only_utf8
+            && expr.properties().look_set().contains(Look::WordAsciiNegate)
+        {
+            return Err(Error::Syntax(
+                "ASCII-only \\B is not allowed in Unicode regexes \
+                 because it may result in invalid UTF-8 matches"
+                    .to_string(),
+            ));
+        }
         // If we're compiling a forward DFA and we aren't anchored, then
         // add a `.*?` before the first capture group.
         // Other matching engines handle this by baking the logic into the